Dc high-voltage generating system

ABSTRACT

A reference resistor for high-voltage application with multilayered insulation films wound around it tightly and which is housed in a grounded metal case. A resistor is vacuum-dried in the case, which is sealed hermetically after being filled with insulation oil dried in vacuum. A DC high-voltage power supply is connected through a filter to the resistor.

United States Patent Saito et al.

on Mar. 18, 1975 DC HIGH-VOLTAGE GEN ERATING SYSTEM Inventors: Yasushi Saito, 663 lchige; Sadayasu Ueno, 1030 lchige, both of Kutsutu, Japan Filed: Dec. 9, 1970 Appl. No.: 96,458

Foreign Application Priority Data [56] References Cited UNITED STATES PATENTS 3.268.797 8/1966 Utsumi et a1. 323/22 V 3,441,894 4/1969 Yzlmumoto et a1. 338/231 Primary E.\'aminerA. D. Pellinen Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT A reference resistor for high-voltage application with multilayered insulation films wound around it tightly and which is housed in a grounded metal case. A resis tor is vacuum-dried in the case, which is sealed hermetically after being filled with insulation oil dried in vacuum. A DC high-voltage power supply is connected through a filter to the resistor.

4 Claims, 6 Drawing Figures JQZ g 525mm 40 J T J' au/v I i "i I I r 1 1 1 1 1 one 5 1 1 I 4/ l :44 q I: J 5/45 f 1 REC77F/E/P 35 L J I i I 45 f i 46 i 1 EEC77F/Ef? Hy C\ LHB l 47 48 EMT? 6E0 7 0 c-- =E-r car/m0; Mi li/Paw C/RCU/T MM 1 DC HIGH-VOLTAGE GENERATING SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC high voltage generation system for use with an electron microscope, particle accelerator, etc. and which requires a highly stabilized operation, or more in particular a reference resistor for high-voltage application as well as its manufacturing method in which a part of aDC high voltage is detected by a voltage-dividing resistor and compared with a reference voltage, the difference between said two voltages being detected, amplified and negatively fed back to a DC high voltage generating section thereby to stabilize the output of said DC high voltage.

2. Description of the Prior Art An electron beam-accelerating voltage for an electron microscope and the like has to be highly stabilized. For example, in an electron microscope whose desired resolving power is 3 A, the ratio of a variation AV of an electron beam accelerating voltage V to the electron rays-acceleration voltage V has to be so small as to meet, the following expression:

AV/V 1 X 10 to l X In a well-known circuit for obtaining such a highly stable voltage, a portion ofa high-voltage output is detected by a voltagedividing resistor and amplified, and it is fed back negatively to a high voltage generation section as a control signal to achieve a stabilization of the output. In other words, a DC high voltage generated by a DC high voltage generation circuit is divided by a reference resistor impressed with a high voltage and by a detection resistor impressed with a relatively low voltage, so that a voltage division across said detection resistor is either fed back negatively to said DC high voltage generation circuit or compared with a reference voltage and the resulting difference is fed back negatively to said DC high voltage generation circuit to stabilize the DC high voltage output.

In the above-described conventional method, the main causes of variations in the DC high voltage output includes noises generated from theresistance material making up the dividing resistor including thermal noises, current noises and other various external noises caused by the application of a high voltage, as well as thermal drifts due to variations in temperature and electric field. Eliminating these sources for unstable voltages or stabilizing a division ratio of said dividing resistor is one of the important factors which has to be emphasized in obtaining a stable DC high-voltage output.

As the detection resistor for this method, a lowvoltage middle-valued resistance material or element is usually employed.'For example, a resistance material or element of 100 K!) to 1 M9 at 100 V or less and with a noise ratio of about 0.1 u V/V is readily available on the market. Since such a detection resistor is used at a low voltage, the electric field generated around the resistance material is weak and an electrostatic shielding is easily accomplished, thereby enabling a voltage induced by external disturbances to be controlled to within 1 ,u. V/V or less.

On the other hand, the reference resistor is impressed with a voltage as high as 50 to 200 kV and has a high resistance valve of 500 to 2,000 MD in order to minimize the power loss at the time of high voltage application. A carbon or metal film is usually employed as a resistance material or element. However, the division ratio of the dividing resistor is subject to variations since a high voltage impressed on the resistor causes a noise current to flow into said reference resistor due to electrostatic induction, thermal noises, thermal drifts attributable to variations in temperature or the transfer of electric charges through the insulating oil.

Further, spaces between the components of the conventional high voltage generating system are arranged taking into consideration the dielectric strength of the system or in such a manner as to protect the system from the influence by electrostatic induction, changes in the temperature around the system and other factors. As a result, it is almost inevitable that the system as a whole becomes bulky.

As a measure to overcome these difficulties, espe- 'cially to control the disturbances due to the highvoltage generation circuit, it has been the practice to contain the reference resistor for high-voltage application in a conductive housing filled with an insulating oil or gas separately from the high-voltage generating circuit, as disclosed in U.S. Pat. No. 3,268,797 issued to Yoshiharu' Utsumi et al., on Aug. 23, 1966.

However, this method still leaves something to be desired in that disturbances which are induced by the transfer of electric charges due to convection of the insulation oil within the case containing the reference resistor for high-voltage application and that the size of the system has yet to be reduced.

'Particularly, the recent demand is for reduction in the size of a high-voltage source for an electron microscope and particle accelerator, that is, a reduction in the space factor of such devices. This requirement for a smaller size, though inconsistent with the need for a more stable source voltage, offersfla problem which has to be somehow solved in this field of the art.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a highly stable, small reference resistor for high-voltage application which can overcome the above-described disadvantages of the conventional system.

Another object of the present invention is to provide a DC high-voltage generating device whose voltage variation is made extremely small by highly stablizing the reference resistor for high-voltage application and the division ratio of a dividing resistor.

To achieve the above-mentioned objects of the present invention, a resistor element with manifold or multilayered insulation films tightly wound around it is contained in a metal case and dried under vacuum. Then it is immersed in insulating oil which is similarly dried in vacuum, and hermetically contained in said case to form a reference resistor for high-voltage application. Further, this resistor, together with other components to which a high voltage is applied, is housed in a metal case filled with the insulating oil to make up a DC high-voltage generation system.

Still other objects and advantages of the invention will be obvious and will be apparent from the specification.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS For fuller understanding of the invention, reference is made to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram showing a section ofthe reference resistor for high-voltage application embodying the present invention;

FIG. 2 is a diagram showing a section of another'embodiment of the reference resistor for high-voltage application according to the present invention;

FIG. 3a is a graph showing a voltage variation curve of a conventional reference resistor for high-voltage application;

FIG. 3b is a graph showing a voltage variation curve of the reference resistor for high-voltage application as shown in FIG. 1;

FIG. 3c is a graph showing a voltage variation curve of the reference resistor for high-voltage application as shown in FIG. 2; and

FIG. 4 is a diagram showing a DC high-voltage generating system for use with an electron microscope to which a reference resistor for high-voltage application of the present invention has been applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Following are causes of noises which are considered to lead to variationsin the division ratio of the dividing resistor and have an effect on the sensitivity of DC high-voltage detection:

1. the l/f noises with a spectrum inversely proportional to the frequencyfwhich are generated from the resistor proper and often much higher than thermal noises within a region of a relatively low frequency in the case of a carbon'resistor (including a carbon film resistor and carbon material resistor) or a metal resistor (including ametal film resistor and metal material resistor). These noises are generally sensitive to the environmental atmosphere and are subject to large variations depending on the gas attached on the resistor element or other conditions of the surface of the resistor element.

2. Noises caused by induced voltages which are in turn attributed to variations in the external electric field because of a high resistance value of the resistor element and a high voltage applied thereto.

3. Since a high voltage is applied to the resistance element, theresistor proper is generally immersed in an insulating oil to improve its dielectric strength. For this reason, a high electric field is formed in the vicinity of the resistor element, so that electric charges are carried from the resistor element to the earth or in the reverse direction by an electrostatic force with the insulating oil acting as a medium, with the result that a noise current flows through the resistor element.

For example, the results of our experiments show that when a DC-voltage of 100 kV is applied between a spherical electrode and a plate electrode placed 20 cm apart in an insulating oil, an average current of about l X A/cm flowed, while variations in the current flowing in the resistor element reached more than 50 percent of the average current.

4. Alternating-current noises of an ultra-low frequency due to thermal drifts were caused by the variations in the temperature around the resistor element. The temperature coefficient of a carbon film resistor for high-voltage application is generally 200 to 600 ppm/C and therefore a noise voltage of 2 to 6 p. V/V is generated due to a temperature change of 001 C.

A reference resistor according to an embodiment of the present invention is constructed as mentioned below to eliminate or reduce the above-mentioned various noises and to obtain a highly stable and small-sized reference resistor for high-voltage application.

Referring to FIG. I which shows a section of a reference resistor for high-voltage application according to the present invention, numeral 1 shows a resistor element to which a high voltage is applied and which consists of, for example, a carbon film resistor element. Numeral 2 is an insulating material of insulating paper or plastic film wound tightly in layers around the entire outer surface of said resistor element 1, and numeral 3 a grounded metal case containing said resistor element 1 with multilayered insulating films wound around it, conducting wires 6 and 7 of the resistor element I being taken out of a metal case 3 through insulating sections 4 and 5 respectively.

A first step of producing the above-mentioned reference resistor for high-voltage application consists of winding insulating films of insulating paper or the like around the entire outersurface of the resistor element tightly in layers. It is recommended that such an insulating film should be as thin as possible, high in dielectric strength and excellent in thermal conductivity. This resistor element is then contained in a metal case 3, whereby a certain space 8 is created between said case and the resistor element with multilayered insulation films would around it. A cooling pipe 9 for supplying cooling water is wound around the outer surface of the metal case 3.

A second step of producing the reference resistor for high-voltage application is to treat in vacuum said resistor element covered with multilayered insulating films, that is to say, to heat the metal case at about 125 C for about 48 hours while evacuating said metal case.

A third step of producing said reference resistor for high-voltage application includes a process of injecting a vacuum-dried insulating oil into the metal case and filling the insulating oil in the spaces between the insulating films without exposing to air the resistor element covered with multilayered films and dried under vacuum. During the process of drying the insulating oil under vacuum, water contained in the oil is removed by repeatedly blowing it out into a heated vacuum container.

After the insulating oil has penetrated into the spaces between said multilayered insulating films to a satisfactory degree, the metal case is filled up with the insulating oil and hermetically enclosed.

On the outside surface of the metal case is wound a tube for supplying cooling water to protect the metal case from influences due to outside temperature changes and to prevent the resistor element 1 from rising in temperature, thereby shortening the time required for the reference resistor to attain a thermal balance. The metal case 3 is grounded to control induced electric charges.

The effect of the present embodiment based on the above-mentioned construction will be explained below. By drying under vacuum the resistor element 1 to which a high voltage is applied, the insulating film 2 and the insulating oil, gases which have been attached to them are removed, while by enclosing them hermetically within the metal case, the environmental atmosphere around the resistor element 1 are much improved thereby to reduce the l/f noises.

Further, the insulation film 2 is wound around the resistor element 1 tightly in layers to such a degree that the insulating oil which has filled up the spaces between the layers of the insulating films is held almost immovable. When, for example, oil-immersed paper is employed as the insulating film, the movement of the insulating oil is prevented by the fibrous substances of the paper and hence noise currents which would otherwise be caused by the movement of electric charges are prevented from flowing into the resistor element, thereby substantially lengthening the insulating distance. In addition, the metal case electrostatically shields the resistor element from the influences of the external electric field.

Further, the resistor element 1 is electrically insulated from the metal case 3 by means of the insulating film and oil to such a high dielectric strength that the distance between the resistor element 1 and the metal case 3 can be maintained smaller and as a result it is possible to markedly reducethe size of the reference resistor.

Another embodiment of the present invention will now be explained with reference to FIG. 2, in which the same components are shown by the same numerals as in FIG. 1. The resistor element 1 to which a high voltage is applied is covered with epoxy resin of 2 to 3 mm in thickness, which, after being tightly covered in layers with the insulating film 2 over the entire outer surface thereof in the same manner as in the embodiment of FIG. 1, is contained in the grounded metal case 3 and dried in vacuum. Then the metal case is filled with the insulating oil and hermetically sealed.

Voltage variations of the device according to the present invention as compared with those of a conventional device are shown in the curves of FIGS. 3a, 3b and 3c, which respectively show the voltage variations of the device disclosed in U.S. Pat. No. 3,268,797, the resistor shown in FIG. 1 and that shown in FIG. 2. They all show the voltage variation AV/V of the resistors im pressed with 75 kV DC of electron beam accelerating voltage, and the abscissa and ordinate respectively show the time in minutes and voltage variations. This is the result of an experiment in which the metal case containing the resistor element has not been controlled in temperature by a cooling pipe. Since the experiments whose results are shown in FIGS. 3b and 30 were carried out under different conditions from the experiment in connection with FIG. 3a, they have developed temperature drifts of 2/1 ,000 C/minute due to changes in room temperature.

Voltage variations in FIGS. 3a, 3b and 3c are respectively AV/V 5 X lO' /minute, AV/V 7 X IO /minute and AV/V 2 X IO /minute. This shows that the resistor element of FIG. 2 which is covered with an epoxy resin film 20 develops a noise voltage about onethird less than the one of FIG. 1 which is not covered with any such epoxy resin film.

In the case of the resistor as shown in FIG. 1, the surface of the resistor element 1 is directly in contact with the insulating oil which permeates the spaces between the insulating films, and therefore, even though the insulating film is wound around the resistor element in layers, the surface of the resistor element 1 is not considered to be always maintained in a uniform condition. The surface of the resistor element 1 of the resistor as shown in FIG. 2, by contrast, is uniformly covered with an epoxy resin film so that it is maintained in a more stable condition, and this is considered to be the reason why less noises are produced or induced in the case of the resistor of FIG. 2.

Generally the voltage variation AV/ V is increased according as the size of a resistor or a metal case containing it is reduced. The size of the conventional reference resistor of FIG. 4 disclosed in U.S. Pat. No. 3,268,797 is 540 mm by 340 mm by 700 mm as compared with the size of the reference resistors as shown in FIGS. 1 and 2 which is mm by mm by 320 mm. In spite of this great reduction in volume to about one-fortieth, the reference resistor according to the present invention develops only about two-fifths of the noise voltage, as compared with the conventional referenceresistor.

A DC high-voltage generation system for use with an electron microscope and to which the reference resistor for high-voltage application according to the invention is applied will be explained below with reference to FIG. 4. Numeral 60 shows an electron gun of the electron microscope which consists of a filament 49, a Wehnelt cylinder 50 and an anode 51 which is usually at ground potential. Numeral 40 shows a power supply for the filament, 41 a power source for a bias to be applied between the filament 49and the Wehnelt cylinder 50, and 42 a power supply for electron beam acceleration by which a voltage of 50 to 200 kV is usually applied between the filament 49 and the anode 51 for acceleration of an electron beam. The above-described filament voltage and bias voltage which are needed to be superposed on the electron-beam accelerating voltage are separated from the earth by means of insulation transformers 43 and 44 respectively. Numerals 45 and 46 show rectifier circuits, 47 an output transformer for the electron beam acceleration power supply and 48 a high voltage rectifier circuit therefor including means for filtering an output of the power supply 42. Symbol R shows the reference resistor for high-voltage application referred to in connection with FIGS. 1 and 2, and symbol r a low-valued detection resistor to which a relatively low voltage is applied.

As mentioned above, it is necessary that a voltage variation AV/V be as small as l X 10" to 10"lminute in an electron microscope. For this purpose, a high voltage output is divided by the reference resistor R and the detection resistor r, so that a voltage appearing across the detection resistor r is compared with a reference voltage V and the resultant difference is detected and amplified by a differential amplifier 70. The output of this amplifier is applied as an input to a control circuit 80 for controlling a high-voltage source output thereby to stabilize the high-voltage output.

High-voltage portions of the above-described component elements are housed in a metal case 52 which is filled with an insulating oil and grounded. Those highvoltage portions such as rectifier circuits 45 and 46 which are contained in the case but separated from the earth are shielded with an electrostatic shield 53 and thereby the distribution of an electric field is regulated to prevent a corona discharge or the like.

Incidentally, a water-cooling pipe may be wound around the outer surface of the metal case 52 for accomplishing a temperature control.

It will be understood from the above description that the high-voltage reference resistor R whose resistor element is contained in the grounded metal case 3 and further contained, together with other high-voltage elements, in the grounded metal case 52 filled with an insulating oil develops less noise voltages.

What is claimed is:

l. A DC high-voltage generating system including a DC highwoltage power supply, filtering means con nected with said DC high-voltage power supply for smoothing the output thereof and voltage stabilizing means for said DC high-voltage generating system, said voltage stabilizing means'comprising:

voltage-dividing means including a high-voltage highresistance reference resistor and a lowresistance resistor to be applied with a relatively low voltage, said voltage-dividing means being connected in series with the output terminal of said DC highvoltage power supply;

a differential amplifier which compares a voltage appearing across said low-resistance resistor with a reference voltage and which detects and amplifies the difference between said two voltages, and a DC control circuit for controlling the DC voltage genessentially consisting of a high-voltage resistor element, an insulating member wound tightly in layers over the entire outer surface of said high-voltage resistor element, anda first grounded metal case filled with an insulating fluid in which said resistor element is disposed, said high-voltage highresistance reference resistor being disposed together with said filtering means in a second grounded metal case which is filled with an insulating oil.

2. A DC high-voltage generating system according to claim 1, in whichsubstantially the entire surface of said high-voltage resistor element is coated with a synthetic resin film several millimeters in thickness.

3. A DC high-voltage generating system according to claim 1, in which a cooling pipe for supplying cooling water is wound around the outer surface of said first grounded metal case thereby to control the temperature of said first grounded metal case.

4. A DC high-voltage generating system according to claim 2, in which a cooling pipe for supplying cooling water is wound around the outer surface of said first grounded metal case thereby to control the temperature of said first grounded metal case. 

1. A DC high-voltage generating system including a DC highvoltage power supply, filtering means connected with said DC high-voltage power supply for smoothing the output thereof and voltage stabilizing means for said DC high-voltage generating system, said voltage stabilizing means comprising: voltage-dividing means including a high-voltage high-resistance reference resistor and a low-resistance resistor to be applied with a relatively low voltage, said voltage-dividing means being connected in series with the output terminal of said DC high-voltage power supply; a differential amplifier which compares a voltage appearing across said low-resistance resistor with a reference voltage and which detects and amplifies the difference between said two voltages, and a DC control circuit for controlling the DC voltage generated by means of an output of said amplifier; said high-voltage high-resistance reference resistor essentially consisting of a high-voltage resistor element, an insulating member wound tightly in layers over the entire outer surface of said high-voltage resistor element, and a first grounded metal case filled with an insulating fluid in which said resistor element is disposed, said high-voltage high-resistance reference resistor being disposed together with said filtering means in a second grounded metal case which is filled with an insulating oil.
 2. A DC high-voltage generating system according to claim 1, in which substantially the entire surface of said high-voltage resistor element is coated with a synthetic resin film several millimeters in thickness.
 3. A DC high-voltage generating system aCcording to claim 1, in which a cooling pipe for supplying cooling water is wound around the outer surface of said first grounded metal case thereby to control the temperature of said first grounded metal case.
 4. A DC high-voltage generating system according to claim 2, in which a cooling pipe for supplying cooling water is wound around the outer surface of said first grounded metal case thereby to control the temperature of said first grounded metal case. 